Hydrothermal quartz on basal plane (0001) and +5.degree.X (rotated +5.degree. from basal plane) seeds has been grown in the laboratory and in production for more than a decade. Systematic studies of the dependence of growth rate on fundamental and engineering parameters have been carried out. Synthetic quartz grown on minor rhombohedral seed plates ("z" or minor rhombohedral quartz sometimes called r-face quartz grown on (1011) seeds) is most economical for the fabrication of AT-cut piezoelectric resonators. These resonators are used in large quantities for monolithic crystal filters and for oscillator plates. Such quartz has been grown for more than 5 years in production.
Experience has shown that synthetic quartz tends to be susceptible to cracking both during the growth process itself, or later in processing the quartz into crystal units. We have established that both of these can result from crystal faults that are propagated into the grown material from faulty seeds. This new and significant observation has indicated a need for attention to seed quality beyond that generally thought necessary.
We have found further that many of these physical defects in the grown material are related to the presence of strains in the seed from which the material is grown.
According to the invention seed crystals used for hydrothermal growth are screened by a novel technique to insure that the seeds are relatively strain free. The screening procedure involves a polariscope examination of the prospective seed crystals in order to detect the kind of gross strains that have been implicated in this work as inducing undesired strains in the grown crystal.
It is well known that strain induces birefringence in transparent crystals and that this birefringence can be seen by passing polarized light through the crystal. Polariscopic examination of quartz to detect twinning and to locate the optic axis in natural quartz has been used for many years. See, for example, G. K. Burns, Bell System Technical Journal, 19, page 516 (1940). For these purposes roughly monochromatic, uncollimated light is used. However, these prior techniques are not sufficiently sensitive to reveal the strains that are of interest in connection with this invention. In order to find strains of this level it is necessary to first, know to look for them, and second, to use the refined polariscopic method now to be described.
To detect the level of strains that are useful in the screening technique of the invention it is necessary to use a plane-parallel sample, to collimate the analyzing light, to direct the analyzing light reasonably precisely along the optic axis, and to use nearly monochromatic light. This will produce a birefringence pattern over the area of the seed crystal that reveals the strain state of the crystal. The birefringence pattern can be evaluated by inspection or can be analyzed quantitatively by appropriate apparatus also to be described.